1. Field of the Invention
The present invention relates to a polyester film and a process for producing the same, and more particularly to a polyester film wherein fine protrusions are formed on its surface and a process for producing the same.
2. Description of the Prior Art
Polyester films are broadly used in various uses. Better surface characteristics such as better running ability, abrasion resistance and so forth have been required for polyester films in order to meet with the increasing speed of film processing, for example, a printing process in use for wrapping films, a magnetic layer applying process used for magnetic recording media, or a thermosensitive layer applying process in use for thermosensitive films, or in order to meet with higher quality requirements for final products. It is known that it is effective to uniformly form fine protrusions on the surface of a film to obtain a good running ability.
With respect to formation of such fine protrusions on the surface of a film, a polyester film containing substantially spherical silica particles originating from colloidal silica is known and described in, for example, JP-A-SHO 59-171623. Further, a polyester film, wherein a thin layer containing particles to form surface protrusions is laminated on a base layer, is also known and described in, for example, JP-A-HEI 2-77431.
In the conventional polyester films such as above wherein protrusions are formed on the film surface by containing particles (for example, inert particles), however, basically there are the following problems.
The most remarkable problem with the formation of protrusions using particles is that, because particles contained are foreign materials relative to polyester, voids (cracks, cavities) are likely to be generated around the particles by stretching the film. Such voids cause various troubles, for example, deterioration of transparency and reduction of dielectric breakdown voltage in use for capacitors. Further, in a base film for a magnetic recording medium, chipping of the surface of the film is liable to occur because protrusions formed on the film surface become brittle. Particularly, in use for a magnetic tape, a base film has been required to be further highly oriented by a higher draw ratio in stretching from the requirement of a higher elastic modulus for the base film. Therefore, troubles such as chipping of the film surface due to generation of voids has become intensified.